1. Field of the Invention
The present invention is directed to a shock wave source of the type having a coil carrier, a coil and a metallic membrane separated from the coil in insulating fashion for generating shock waves.
2. Description of the Prior Art
Electromagnetic shock wave sources of the type described above are utilized, for example, in medicine for the non-invasive disintegration of body calculi of a patient, for example for the disintegration of kidney stones. The generation of shock waves with such a shock wave source is accomplished by a brief-duration high-voltage pulse applied to the coil arranged on the coil carrier. As a result of the electromagnetic interaction of the coil with the metallic membrane separated from it in insulating fashion, the membrane is repelled from the coil in an enclosed volume of water located between the shock wave source and the patient. As a result, attenuated sine waves are emitted into the water as carrier medium between the shock wave source and the patient. Shock waves ultimately arise due to non-linear effects in the carrier medium, water. The attenuated sine oscillations have a basic frequency of about 150 through 200 kHz that is determined by the electrical properties of the shock wave source. The sine waves lie outside the human hearing range.
Nevertheless, audible waves arise when generating shock waves with an electromagnetic shock wave source—as described, moreover, in structure and function in, for example, H. Reichenberger, G. Naser, “Electromagnetic Acoustic Source for the Extracorporeal Generation of Shock Waves in Lithotripsy”, Siemens Forschungs-und Entwicklungsberichte, 15, 1986, No. 4, pages 187 through 194. Simultaneously with the emission of the sine waves into the water path, waves propagate in the opposite direction in the coil carrier (usually formed of ceramic) that can convert initially axially propagating waves into radial or plate waves. The radial or plate waves cause the coil carrier to oscillate such that low-frequency waves arise in the human hearing range, i.e. below 20 kHz. Due to the highly symmetrical geometrical shape of the coil carrier (the coil carrier usually has a circular cross-section in planes at a right angle to its longitudinal axis) equiphase superimpositions of radial and plate waves also occur due to reflections at the edge of the coil carrier. As a result, audible waves arise that have a very unpleasant sound level for patients and medical personnel.